The Beckmann rearrangement reaction, which consists in converting ketoximes to the corresponding substituted amides by means of acidic reagents, has been known for a very long time.
This reaction is taken advantage of in the industrial production of lactams from cyclic ketoximes, more particularly in order to form caprolactam and lauryllactam, which are base monomers of polyamide-6 and polyamide-12 respectively.
Provision has been made for the use of various acidic reagents in carrying out the Beckmann rearrangement.
The use of sulphuric acid, alone (see DE-8-15 45 653 and FR-A-2 417 501) or as a mixture with trifluoroacetic acid (see JP-A-51034185) or sulphur trioxide and chlorosulphonic acid (see JP-A-57031660).
Provision has been made for the use of phosphoric acid (see CH-A-530402 and JP-A-62149665) or polyphosphoric acid (see DE-B-1 545 617).
The use has also been disclosed of acetic acid (see CH-A-394212), of a mixture of acetic acid and cyanuric acid (see JP-B-71023740), of a mixture of acetic acid and acetone (see JP-A-51004163), of a mixture of acetic acid, acetone and a fluorinated catalyst (see JP-A-51004164) and of a mixture of acetic acid or acetic anhydride and hydrofluoric acid (see U.S. Pat. No. 3,609,142).
Finally, provision has been made for the use of hydrochloric acid in conjunction with a polar organic solvent (see DE-A-1620478) or with a catalyst, for example a-metal salt (see U.S. Pat. No. 3,904,608) or a mixture of silica and alumina.
Sulphuric acid is by far the most commonly employed acid on an industrial scale. However, sulphuric acid is not without disadvantages.
It is known that, under the temperature conditions of the rearrangement (greater than 135.degree. C.), sulphuric acid is a factor which promotes the appearance of hydrolysis side reactions. This hydrolysis takes place on the starting cycloalkanone oxime, which it converts to ketone, on the one hand, and on the final lactam, which it converts to amino acid, on the other hand. This results in a decrease in the production of lactam and additional difficulties in the subsequent stages of separation and purification of the lactam.
When the oxime to be treated comprises a residual chlorinated solvent originating from the preceding stage, as is in particular the case when photonitrosation of cycloalkane is involved, two side reactions appear.
The first reaction leads to partial decomposition of the sulphuric acid with release of sulphur dioxide. During the various operations of recycling the organic phase comprising the unreacted cycloalkane, the content of sulphur dioxide increases, the effect of which is to slow down the photonitrosation reaction.
The second side reaction causes decomposition of the residual chlorinated solvent to phosgene, which is toxic to man.
Finally, all the effluents comprising sulphuric acid which are generated by the industrial process can only be recycled at the price of a lengthy, difficult and expensive treatment.